The eruption of teeth from their place of development within bone to their place of function within the oral cavity is a complicated process. Recent experiments have shown that the periodontal ligament, which attaches the tooth to alveolar bone, is capable of providing a continuing eruptive force once the bone over the tooth bud has been removed. Other experiments suggest that formation of the eruptive path in bone by resorption is coordinated with, but not dependent upon, tooth eruption. Little is known of the role of bone resorption in tooth eruption or the factors involved in this localized resorption of alveolar bone. The relationships between tooth eruption and bone resorption will be examined in an experimental animal, the ia rat, in which bone resorption is reduced and teeth do not erupt. The cells responsible for bone resorption, the osteoclasts, can make but not release membrane-bound packages of acid phosphatase, an enzyme associated with bone removal. Bone resorption can be increased in these animals if a source of competent osteoclasts is provided by transfusion of spleen cells from a normal littermate. The effect of splenic transfusion at birth on the cellular differentiation of osteoclasts and on eruption of the dentition in ia rats will be determined. This study is expected to elucidate the importance of the osteoclast in tooth eruption and the source and differentiation of the cellular source of bone resorption, the osteoclast. The results have application in determining ways to reduce and reverse the localized loss of alveolar bone in periodontal disease.